Moore was initially meant to play the main character in the film, literary forger Lee Israel, while co-writer Nicole Holofcener was going to direct.
The 58-year-old actor was fired by Holofcener two days before filming commenced, following a row over “creative differences”.
Speaking at Advertising Week, Grant explained what those differences were.
“Julianne Moore wanted to wear a fat suit and a false nose to play Lee Israel and [co-writer] Nicole Holofcener said, ‘You’re not going to do that’,” the Oscar-nominated actor said.
Grant – who played the character Jack Hock, Israel’s friend, in the film – added that Helena Bonham Carter and Sam Rockwell were originally going to play the two central roles, with Bob Balaban directing.
“Fast forward 10 years and then it was going to be Julianne Moore and Chris O’Dowd and then Nicole Holofcener, the co-screenwriter, fired Julianne Moore two days before they started shooting over creative differences,” Grant continued.
Speaking previously about leaving the project, Moore said: “I didn’t leave that movie. I was fired... Nicole fired me. I think she didn’t like what I was doing.”
“I think that [Holofcener’s] idea of what the character was was different than what my idea of the character was, and so she fired me,” Moore added.
Melissa McCarthy, who ended up playing Israel, and Grant were nominated for Oscars at the Academy Awards ceremony earlier this year for their roles in the film. Holofcener was also nominated for Best Adapted Screenplay. Marielle Heller directed the film.